Brake disks made of fiber-reinforced ceramic designed as an annular disk fastened to a hub are usually composed of multiple layers, whereby the outer layers which come into contact with the brake shoes are to be optimized with respect to their tribological properties, whereas the inner layer or layers have essentially mechanical functions (transfer of the brake torque to the axle) and thermal functions (heat dissipation). The same tribological and mechanical functions must also be met by a clutch disk. The materials are correspondingly optimized for these functions.
German Unexamined Patent application DE 100 48 012 A1 describes carbon-ceramic brake disks having friction layers containing carbon fibers in the form of bundles with lengths of 0.25 mm to 8 mm, the bundle widths of the carbon fiber bundles being between 0.001 mm and 1.5 mm. Carbon-ceramic brake disks are known from DE 101 18 921 A1, for example, having at least a three-layer structure, the two outer layers interacting with the brake shoes as friction layers and thus achieving the tribological function, and at least one inner layer having ribs with a radial or involute curvature and separated by cavities designed as channels; whereby air passes through these channels and assists in heat dissipation, the inner layer being made of fiber-reinforced ceramic material which contributes to mechanical stability.
In accordance with these various functions, for the friction layer (tribological function) and for the support body (mechanical function) fiber-reinforced ceramic materials are used which differ in the type and quantity of reinforcing fibers. Long fibers having lengths of greater than 6 mm contribute in particular to mechanical stability, whereas short fibers having lengths of less than 6 mm are preferably used for friction layers. The fiber content in the friction layers is generally less than in the support bodies, which are subjected to mechanical stress.
According to the teaching of DE 100 48 012 A1, fiber fractions having lengths in the range of 0.25 mm to 8 mm are used for the friction layer, whereas fiber fractions having lengths in the range of 4 mm to 20 mm are used for the support bodies. These fractions are obtained from ground fiber bundles, in this case the fiber bundles being produced from bundled continuous fibers (fiber cable) by impregnation with resins or pitch as binder, carbonization of the binder, and, if needed, repetition of the impregnation and carbonization. On account of the grinding step, the length as well as the thickness of the fiber bundles are subject to statistical fluctuations which result in nonuniform material properties, particularly with respect to the tribological properties, whereby the fluctuations in the material properties may be intensified by further processing, for example.
The object, therefore, is to improve the tribological properties of friction layers made of fiber-reinforced ceramic by using fiber bundle functions having the maximum possible homogeneous or narrowly distributed bundle lengths and bundle widths. It is also desirable to further improve the mechanical properties of the support bodies.
Polymer-bound fiber tows are known from EP 1 645 671 A1, having an average length of 3 mm to 50 mm measured in the fiber direction, and an average bundle thickness of 0.1 mm to 10 mm measured perpendicular to the fiber direction, and in which at least 75% of all fiber tows have a length that is at least 90% and not greater than 110% of the average length.
It has been found that such polymer-bound fiber tows may be advantageously used for manufacturing carbon-ceramic brake disks, and in the production of friction layers as well as the support bodies for carbon-ceramic brake disks. The referenced polymer-bound fiber tows may also be advantageously used for manufacturing clutch disks of carbon fiber-reinforced ceramic (carbon-ceramic clutch disks).